A continuously variable transmission of this kind is constructed with clamping a disc-shaped roller between a pair of discs arranged opposed to each other. On the outer portion of the opposed face of the pair of discs from the predetermined radius, there is formed a rolling face which has an arcuate shape centered on the point set in between the opposed faces of the pair of discs. This rolling face is leading to circumferential direction of each disc. Thus, the rolling face formed on each disc and incurved in three-dimensional direction is a toroidal plane. In between those rolling faces, a rolling member is clamped in a rotatable condition.
This rolling member is a disc-shaped member and its sectional figure along the in-depth direction of the outer circumferential portion in a plane including the axis of rotation is congruent with the arcuate figure of the rolling face of the disc. Accordingly, the rolling member is rotated by means of rotating one of the discs, and the other disc rotates subsequently. And, a speed change ratio in accordance with the proportion of the contacting radii is set by means of adjusting contacting radius between the rolling member and one of the disc, and contacting radius between the rolling member and the other disc, with inclining the rolling member.
One example of such toroidal type continuously variable transmission is disclosed in Japanese Patent Laid-Open No. 2000-507667. The toroidal type continuously variable transmission disclosed in this Japanese Patent Laid-Open comprises an input shaft and an output shaft arranged in parallel each other. There are arranged two each of input discs and output discs on the input shaft side. Two of input discs are arranged in an axial direction at predetermined interval, and two of output discs are arranged in between those input discs. Two of output discs are constructed integrally with having its rolling face positioned in both sides of the axial direction. And there is provided the rolling face on the opposed face of the input discs and output discs respectively.
Also, one of the input disc and input shaft are constructed with being allowed to move relatively in the axial direction and to rotate integrally. On the other hand, another input disc and input shaft are constructed impossible to move relatively in the axial direction, but possible to rotate integrally. One of the input shafts is arranged inside of a cylinder, and a hydraulic chamber is formed in the cylinder. On the other hand, two of output discs and the input shafts are constructed with being allowed to rotate relatively, and there is provided a chain driveline for transmitting torques of two output shafts to the output shafts. The rolling member is arranged between the input disc and the output disc, and the rolling member is clamped by the rolling faces of each disc.
The torque of the input shaft is transmitted to the output disc via the input disc and the rolling member, then, the torque transmitted to the output disc is transmitted to the output shaft via the chain driveline. The principle of transmission of the torque will be described specifically hereafter. The facial pressure of the contact points between the roller and each disc become high pressure, and the torque is transmitted by shearing resistance of lubricating oil being interposed between the rolling member and each rotary member. It is so-called traction transmission. In short, the torque capacity between the input disc and the output disc is changed according to thrust force of each disc against the rolling member. And according to the continuously variable transmission disclosed in the above-mentioned Laid-Open, the clamping force of each disc is controlled by controlling an oil pressure of a hydraulic chamber which is formed inside of the cylinder.
As mentioned above, transmission of the torque in the continuously variable transmission is executed according to the load (pressure) to thrust each disc and the rolling member. Therefore, the bigger torque to be transmitted becomes, the more clamping force for clamping the rolling member increases. In consequence, the amount of heat generated between each disc and the rolling member becomes bigger, thereby the rolling member intends to expand. However, since the clamping force is applied to each disc in the axial direction and expansion of the rolling member is thereby prevented, a heat stress is increased inside of the rolling member. As a result, the facial pressure between each disc and the rolling member becomes higher needlessly, i.e., higher than the facial pressure according to the torque to be transmitted. Accordingly, there is a possibility to deteriorate duration of the life of the rolling member. According to the invention disclosed in the above-mentioned Laid-Open, however, there is no recognition of the relation between clamping force in the axial direction to be provided for each disc and the temperature. Accordingly, there is room for improvement with respect to this point.
This invention has been conceived noting the aforementioned technical problem and has an object to provide a toroidal type continuously variable transmission which can improve the endurance of the rolling member to be clamped between the input disc and the output disc.